-I would push it up to 1024 to see if it levels out.
Watch for CPU spikes if there is any in the task manager/process tab.

-If you think it may not be the audio driver, it could also be an expired plugin, I know the Coyote Forte DXI plugin when expired will introduce an interrupt in the music (like clockwork) roughly every 10 seconds. So it may be an expired plugin of some sort.

-The other item I would look at is laptop specific, make sure the control panel/power options is set to a high performance power scheme.

-If necessary you can also visit the RealTracks settings dialog (if your issue tends to happen with RealTracks) and if you have high quality tempo/pitch stretching enabled you and clear that check box temporarily to see if it helps any.